1. Field of the Invention
The present invention relates to a semiconductor device to be used as a resistance element in an integrated semiconductor circuit and a method of fabricating the same. Particularly, the present invention pertains to a semiconductor resistance element suitable for miniaturization and a method of fabricating the same.
2. Description of the Related Art
FIG. 1A is a plan view illustrating a conventional semiconductor resistance element, and FIG. 1B shows a cross section taken along the line III--III of FIG. 1A.
An N.sup.+ -type diffusion layer 22 is selectively formed on the surface of a P-type silicon substrate 21. On the P-type silicon substrate 21 including the N.sup.+ -type diffusion layer 22 is formed an N-type epitaxial layer 24 doped with phosphor or the like of 10.sup.15 to 10.sup.16 atoms/cm.sup.3. In other words, the N.sup.+ -type diffusion layer 22 is embedded between the P-type silicon substrate 21 and the N-type epitaxial layer 24. An oxide-film insulating region 25 which reaches the P-type silicon substrate 21 is selectively formed on the surface of the N-type epitaxial layer 24. The N.sup.+ -type diffusion layer 22 and the N-type epitaxial layer 24 immediately atop the layer 22 are therefore isolated from the neighboring element(s). A P.sup.+ -type diffusion region 23 is selectively formed on the surface of the P-type silicon substrate 21 immediately underlying the oxide-film insulating region 25 in order to prevent inversion between elements. P.sup.- -type resistance regions 27a to 27c are selectively formed on the surface of the N-type epitaxial layer 24 immediately on the N.sup.+ -type diffusion layer 22 by implanting ionized boron or the like of 10.sup.17 to 10.sup.20 atoms/cm.sup.3. The P.sup.- -type resistance regions 27a to 27c are rectangular from the plan view, and formed almost in parallel to one another. An insulating film 26 is adhered to the N-type epitaxial layer 24. Contact holes 28 are selectively formed in both end portions of each of the P.sup.- -type resistance regions 27a to 27c. Patterned on the insulating film 26 are a wiring 29 to be connected via one of the contact holes 28 to one end of the P.sup.- -type resistance region 27a and a wiring 30 to be connected via the other contact holes 28 to the other end of the resistance region 27a and the ends of the resistance regions 27b and 27c on the same side.
In the above-structured conventional semiconductor resistance element, the P.sup.- -type resistance regions 27a to 27c are isolated insulatively from the N-type epitaxial layer 24 by a depletion layer of a PN junction, thus acting as a resistance element. For example, if the boron concentration is 10.sup.17 atoms/cm.sup.3, the P.sup.- -type resistance regions 27a to 27c will have its sheet resistance .sigma..sub.s of about several tens k.OMEGA./.quadrature.. When the boron concentration is 10.sup.20 atoms/cm.sup.3, the sheet resistance .sigma..sub.s will be about 100 .OMEGA./.quadrature.. In other words, the lower the boron concentration is, the more the depletion layer tends to extend toward the P.sup.- -type resistance regions 27a to 27c. The effective sheet resistance .sigma..sub.s therefore will be larger.
The resistance R of the P.sup.- -type resistance regions 27a to 27c is represented by the equation (1). EQU R=.sigma..sub.s .multidot.L/W+r.sub.0 ( 1)
where L is the length (.mu.m) between the contacts of the P.sup.- -type resistance regions 27a to 27c, W is the width (.mu.m) of the P.sup.- -type resistance regions 27a to 27c, and r.sub.0 is a lead contact resistance (.OMEGA.).
The conventional semiconductor resistance element described above is, however, a planar electric resistance element. In order to acquire a high resistance R with a low sheet resistance .sigma..sub.s, the resistance length L should be increased. In the case of acquiring a high resistance R with a high sheet resistance .sigma..sub.s, since the amount of boron to be implanted tends to vary, the resistance width W should be increased so as to restrict a change in resistance R caused by the depletion layer. The semiconductor resistance element is therefore hard to be miniaturized.